Changes in Outstanding Common Stock (Details) - shares shares in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity [Abstract]
Outstanding, beginning of year (in shares)	99,843	99,377	99,204
Shares issued upon exercise of options (in shares)	416	125	104
Shares issued for time vested restricted stock (net of cancellations) (in shares)	952	341	69
Shares issued for acquisition (in shares)	3,361	0	0
Outstanding, end of year (in shares)	104,572	99,843	99,377